1. Field of the Invention
This invention pertains to a travel drive apparatus for hydraulic drive work vehicles and a control method therefor, and more particularly to a travel drive apparatus for hydraulic drive work vehicles, and a control method therefor, designed for a hydraulic drive work vehicle that comprises a variable delivery hydraulic pump for traveling that is driven by an engine, which vehicle travels as a drive wheel or wheels are driven by a hydraulic motor that is acted on by hydraulic pressure from the hydraulic pump, which apparatus prevents cavitation from occurring.
2. Description of the Related Art
Variable delivery hydraulic pumps (hereinafter called hydraulic pumps) are conventionally used as pumps for generating oil under pressure for driving hydraulic shovels and other working machines or traveling equipment. These hydraulic pumps are equipped with horsepower control mechanisms (hereinafter called regulators) to prevent the engine that drives the pump from stalling. Such a regulator controls the flow rate Q in response to the discharge pressure P, so that equipment is run with roughly constant torque (P.times.Q=constant). When the discharge pressure P is low, the force generated by a piston (not shown in the drawings) is smaller than the force of an opposing spring, wherefore the piston does not move, so that the cylinder block of the pump is in the position of maximum tilt angle, and the pump discharge quantity is also at maximum. When the load acting on the pump, that is, either the load of the working machine or the load when traveling, increases, the piston moves to a position that balances the force of the spring, decreasing the cylinder block tilt angle, and control is effected so that torque becomes constant. As described above, the spring used in the regulator pushes the cylinder block in the direction of maximum tilt angle. Alternatively, in another known example, the spring used in the regulator pushes the cylinder block in the direction wherein the tilt angle is minimized. Thus, when the engine is started, the load driving the pump becomes small, making engine startup easy.
Also known are hydraulic drive apparatuses, which use hydraulic pumps and hydraulic motors, for enabling hydraulic shovels and other construction equipment to travel. Among these known hydraulic drive apparatuses are those wherein the hydraulic pumps and hydraulic motors are connected in a closed circuit, and those wherein the hydraulic pumps and hydraulic motors are connected in an open circuit with changeover valves inserted between the hydraulic pumps and hydraulic motors. An example of such an open circuit is disclosed in Utility Model Registration No. 2543146, in gazette. According to this model, as diagrammed in FIG. 4, this hydraulic circuit comprises a hydraulic pump 201 for driving various actuators in the work vehicle, compound control valves 202A and 202B that are a collection of control valves for controlling the supply of pressure oil from the hydraulic pump 201 to each of various actuators, and a travel hydraulic motor 204 that is drive-controlled by travel control valves 203a and 203b for the compound control valves 202A and 202B. To a main line 205 for the travel hydraulic motor 204 are connected crossover relief valves 206, counterbalance valves 207, and lower makeup valves 208. The lower makeup valves 208 and an oil tank 210 are connected by a makeup circuit 212, and an oil cooler 211 is provided in the return oil line of the compound control valve 202A. A center joint CJ is also provided for circulating oil between an upper revolving structure and a lower revolving structure.
As based on the present invention, furthermore, while one end of the makeup circuit 212A is connected to the main lines 205A and 205B via a lower makeup valve (second makeup valve) 208, the other end thereof is connected to a line 222 upstream of a cooler relief valve 221 via the makeup circuit 212B. The upstream line 222 is the return line for the compound control valves 202A and 202B. Also, upper main lines 205C and 205D between the center joint CJ and the travel control valves 203a and 203b built into the compound control valves 202A and 202B are connected to the makeup circuit 212B via an upper makeup valve (first makeup valve) 223, and makeup oil is replenished from various portions to the upper main lines 205C and 205D. A hydraulic pump 201D is a hydraulic steering pump, connected to a hydraulic steering cylinder 225 via a steering valve 224. The return oil from the steering valve 224 is connected to the upstream line 222 of the cooler relief valve 221 via the makeup circuit 212B. Thus makeup oil is introduced to the upper main lines 205C and 205D on the side of the upper revolving structure that connects the center joint CJ and the compound control valves 202A and 202B. Accordingly, if a large flow rate of makeup oil is introduced, a large flow rate of makeup oil will be conveyed all the way to the travel hydraulic motor 204 installed in a lower traveling body. Also, the return oil from the hydraulic steering pump 201D is merged into the makeup circuit 212, wherefore an adequate makeup flow rate can be secured. Accordingly, cavitation can be definitely prevented in the travel hydraulic motor 204. In addition, makeup oil is also replenished directly to a lower main line from the makeup circuit 212. The pressure of the makeup oil can be set by the cooler relief pressure, and it is stated that replenishment efficiency is improved.
With a hydraulic drive apparatus for effecting travel in a hydraulic shovel or other construction machine, however, there are problems. Namely, when decelerating, descending a slope, or changing either from forward travel to reverse travel or from reverse travel to forward travel, cavitation occurs which damages the hydraulic motor, and, when descending a slope, due to overrun, the vehicle ceases to be controllable. Therefore, when a closed circuit configuration is used, in order to prevent overruns, the inertial energy of the working vehicle (roughly 125% of the rated vehicle speed) must absorb the inertial energy generated by the reverse drive torque of the engine. Hence, in order to absorb this with the travel hydraulic pump and travel hydraulic motor, the capacity (discharge volume per revolution, in cc/rev) has to be made large. With the closed circuit configuration, moreover, oil is supplied from a charge pump on the intake side of the closed circuit so that cavitation does not occur, and the supply volume of this charge pump must also be made large. Thus, the charge pump drive force must become large, and the engine output horsepower must be increased. The engine will become large, and wasted energy will be developed when traveling normally. The maximum traveling speed is determined by the capacities of the hydraulic pump and hydraulic motor, wherefore, at the very least, it is necessary to use a hydraulic pump having large capacity from the outset. In a large working vehicle, a hydraulic pump having larger discharge volume will become necessary, and, together therewith, it will be necessary to increase the engine output horsepower, which is uneconomical.
Next, according to Utility Model Registration No. 2543146, in gazette, configured with an open circuit, makeup oil is replenished from the makeup valve when normally moving forward, moving in reverse, accelerating, or descending a slope, and cavitation is prevented. However, when changing from forward travel to reverse travel or from reverse travel to forward travel, cavitation develops, damaging the hydraulic motor, and rendering control of the vehicle impossible. Suppose, for example, that an operator moves travel control valves 203a and 203b from the forward position (A) past neutral to the reverse position (C), thereby changing the vehicle from forward travel to reverse travel. While moving forward, pressure oil passes through the upper main line 205A and reaches the intake 204A in the travel hydraulic motor 204, turning the travel hydraulic motor 204, and moving the vehicle forward. When reverse travel is changed to, the pressure oil, from the reverse position (C) of the travel control valves 203a and 203b, passes through the upper main circuit 205D, reaches the counterbalance valve 207, and switches the counterbalance valve 207 to the reverse position (C). The pressure oil from the hydraulic pump 201, from the counterbalance valve 207 in the reverse position (C), passes through the lower main circuit 205B, reaches the intake 204B in the travel hydraulic motor 204, and tries to turn the travel hydraulic motor 204 and make the vehicle move in reverse. At this time, the travel hydraulic motor 204 is still turning in the forward direction due to the inertial energy of the vehicle, and oil is being discharged from the intake 204B in the travel hydraulic motor 204. For this reason, the pressure oil from the hydraulic pump 201 and the oil from the travel hydraulic motor 204 are discharged to the lower main circuit 205B and put under high pressure, whereupon the crossover relief valve 206 is activated. With the oil from this crossover relief valve 206, makeup oil is replenished to the lower main circuit 205B via the upper makeup valve (first makeup valve) 223, and cavitation is prevented. At this time, however, the oil discharged from the hydraulic pump 201 is at high pressure, wherefore, in the conventional hydraulic circuit, a regulator is used to reduce the discharge quantity of the hydraulic pump, so makeup oil is not adequately replenished to the lower main circuit 205B, and cavitation occurs. When the inertial energy of the vehicle is large, the discharge quantity of the hydraulic pump will become smaller while the crossover relief valve 206 is operating, cavitation will occur, and the vehicle will not stop within the designated range, which is a problem.
In view of the problems set forth above, an object of the present invention is to provide a travel drive apparatus for hydraulic drive work vehicles, and a control method therefor, having a simple configuration, wherewith it is possible to prevent the cavitation that predictably occurs when the work vehicle decelerates, descends a slope, or is switched either from forward travel to reverse travel or from reverse travel to forward travel.